Molten salt spray apparatus for descaling stainless steel



J- A. FALER March 23, 1965 MOLTEN SALT SPRAY APPARATUS FOR DESCALINGSTAINLESS STEEL Filed Oct. 23, 1963 R m 2 m G W T F E T L L "I. N m M mm M A A n m m E s E m 7 T s M 3 s 7 A 2 4 7 3 3 X I r a 9 6 2 L 4 Q a 3Q w x14 \1 1/ JOHN A. FALER ATTORNEY United States Patent 3,174,491MULTEN SALT SPRAY APPARATUS FOR DESALKNG STAKNLESS STEEL John A. Faler,Detroit, Mich assignor to Knlene Corporation, Detroit, Mich. Filed Oct.23, 1963, Ser. No. 318,2% 2 Claims. (Ci. 134-1492) Stainless steel hasbeen cleaned successfully in immersion baths. See Dunlevy UnitedStatesPatent No. 2,635,- 062, of April 14, 1953, and Webster United StatesPatent No. 2,458,661, of January 11, 1949. The cleaning operationremoves oxides and gives an excellent finish to the material, bright,high bufling quality, and high corrosion resistance.

This operation is more than a cleaning operation; it is a process whichconditions the stainless steel strip by oxidizing scale, after which thethus oxidized scale may then be processed in accordance with the processof United States Patent No. 2,458,661 or any other process as, forexample, through acid pickling, etc.

However, it was found that for various reasons, scratching and mar-ringof the strip occurred, explained because of the presence of insolubleparticles on the strip and also because of the transverse movements ofthe strip on the metal'rolls. Rolls are needed to guide the stripthrough the bath, and metal has to be used for the rolls.

In addition to scratching and marring of the strip because of particleson the strip and transverse movements of the strip under metal guiderolls, or over metal guide rolls, another important fact to consider isthat the more rolls used for starting strip under or over the metalguide rolls, the greater the amount of tension. This results in anactual elongation of strip under some conditions, or a certain amount ofdistortion, sometimes causing socalled cross breaks which run across thesurface of the strip, or irregularities lengthwise, such as depressionsor raised surfaces due to this tension. It is desirable to eliminaterolls that are used to guide strip and, theoretically, so the pass linecould be straightwith no dipping into any tanks, Whether itbe salt,Water, acid or other solutions.

As a result, the immersion bath process is limited, by commercialfactors, to intermediate grades of stainless steel. When it came tocleaning of very high quality stainless steel, the immersion bathprocess has not been widely adopted and instead, processors for thesehigher grades of stainless steel continue to resort to other'and oldermethods of cleaning, such as acid treatment.

The immersion process also was not widely adopted for very high qualitystainless steel, nor extremely light gauges of stainless steel (such asbelow .010), particularly finished product where no additional coldrolling is used.

Hence, there has been developed and here disclosed a new process .forcleaning stainless steel, and this process is the subject matter of thisapplication. In the process, the stainless steel instrip form comes outof an annealing oven and into a spray oven or muffle or box where it iscleaned or oxidized by molten salt spray and then emerges into thecustomary treatments such as water rinsing, quenching, dilute acidpickling, rinsing, and then rewinding. The novelty here is the sprayoven and what happens in the spray oven where molten salt spray isthrown onto the strip as it leaves the annealing furnace.

The spray mufiie or oven under consideration comprises an enclosed sprayzone which in practical structural form may be considered as a box intowhich the hotstrip enters at one end from the annealing furnace and fromwhich the oxidized or cleaned strip emerges at the other end, thencepassing into a rinsing zone which may be a rinsing bath or water spray.

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The oxidizing oven or spray box, as we may call it, receives the hotsteel stripfrom the annealing furnacexand discharges it to the rinsingbath. In the oxidizing oven, there are no rolls to be engaged by themoving strip but the strip is moved freely under tension through theoven, entering a slot in one end and leaving through a slot in the otherend and at no time in theoxidizing oven is'the strip in contact with anyrolls or supports or guides.

The strip is not in contact with metal rolls as it passes through thespray box. Hence there is no electrolytic or no galvanic action, aswould be encountered in an immersion bath containing salt and metalrolls over which a metal strip passes.

The hot steel strip enters at a temperature of about 1000 F. and emergesat about the same temperature, there being no substantial drop intemperature of the strip until it enters the water rinse and quench. Itis important to maintain the strip at substantially the same temperatureall the way through the oxidizing oven. As long as the strip is at hightemperature, in the presence of a moiten salt spray, the oxidizingaction will take place without the formation of any solid particles thatmight otherwise abrade or scratch the strip. The strip enters hot and ismaintained at substantially the same temperature all the way through theoxidizing oven.

The oxidizing action is obtained by subjecting the strip and theatmosphere within the oven to a molten salt spray, such as Kolene No. 1,see United States Patent No; 2,458,- 661, for example, and the spray isformed byatornizing the molten salt, using large quantities of an inertgas such as superheated steam at about 1000 R, which passes intonozzles, into which is fed in very tiny quantities and in smallincrements the molten salts which, at that point, is at a temperatureof1000 F. The exact temperature is not critical, except that it exceedthe melting point of the salt spray, about 550 F., a substantial rangebeing permitt'ed.

The-interior of the oven is also maintained charge'diwith an atmosphereof superheated steam coming from different outlets from the steamsuper-heater. In other words, some of the steam goes out through steamnozzles into the atmosphere for heating the atmosphere and filling itfull of steam, and another part of the steam goes through the atomizingnozzles.

The steam maintains a hot steam atmosphere in the oven so that in theevent of a possibility of some of the salt coming through in unatomizedcondition from the nozzles, then this salt will be carried around by thesteam and be maintained molten by the steam so that at no point can anysolidified salt particle engage the strip as it goes through the oven.

Another important reason for loading the oven atmosphere with steam isto prevent the possibility of any gas, such as air, entering the oven.Steam is inert tothe salt. Other atmospheres, such as air, would reactwith the salt and form solid particles which would impair the operation.By using steam, or similar inert gas, we insure against anything elsecoming into the oven, and-the steam coming in under superheatedconditions seals the slots through which-the strip is passing andprevents air from entering the oven at such slots. Otherwise, the ovenis maintainted sealed whenever possible, and even where there are breaksin the seal, the fact that "the oven is loaded with steam seals thesebreaks against the entrance of air into the oven from outside.

The pressure of 1000 F. steam is greaterthan thepressure of theatmosphere outside of the oven. It seems that when atmospheric air comesin contact witha heated steel surface, certain higher forms of oxidesare promoted. Since it is a desire and intention, by the use'of'themolten salt spray process, to form no oxide or a form of oxide moreeasily soluble, it would appear that, since the pressure of steam doesnot allow atmosphere to enter the oven, while the strip is hot, thelower forms of oxide formed in the presence of steam with the exclusionof air is more desirable, and this is another attribute of the moltensalt spray process.

In addition, to insure the temperature within the oven remaining at thedesired 1000 F., and in order to provide a safety factor in the eventthe steam does not function, additional heating means may be employed tomaintain the atmosphere at the desired temperature. Such additional orauxiliary heating means may or may not be used in the actual operationfrom time to time, but is initially provided as equipment so that it isavailable when needed. This additional means could comprise combustionburners for discharging large quantities of hot combustion vapors into ajacket around the oven, and such jacket is sealed from the oven so thatno combustion vapors can enter the oven, and the oven is maintained freeof any atmosphere except inert gas, such as steam.

The auxiliary heating means, of course, could be electric heaters or anyother type of heater, just so long as it heats from the outside and doesnot discharge any harmful vapors into the oven. If air or combustionvapors were to enter the oven, harm might result. The air or vaporscontain CO and because CO by reaction with the salt may form carbonates(CO and because these carbonates are solidified, there might result aplugging of the nozzles through which the molten salt is atomized and,also a marring of the strip by the solid particles.

The superheated steam moves the atomized salt spray with very highvelocity onto the strip and by so doing, concentrates the scouring andchemical action of the molten salt onto the strip and, thus, reducespossible wastage of molten salt. It is estimated that at least 90% ofthe molten salt is actually used in the reaction that takes place of thesalt on the scale and converts it into oxides, and no more than isunused in the process.

The salt spray also has a scale loosening, scouring, or scrubbingaction. Impinging on the strip, it facilitates oxidizing of the scale.

At the discharge end, and preferably at the lowest point of thedischarge end, there is a condenser through which is exhausted steam andexcess salt vapors. The condenser removes and exhausts the oven, whichis constantly receiving a supply of fresh salt and fresh steam, and atleast 90% of the salt enters into the chemical change.

The system operates in such a way that any scale or sludge formed isremoved in the form of vapors suspended in the steam and exhaustedthrough the condenser and the exhaust system.

The dragout losses are extremely small here because instead of having amolten salt film of excess amounts on the strip, as is conventional withbath type processes, such excess molten salt film being needed tolubricate the strip as it passes over the rolls, here we use only asmuch salt as is absolutely necessary to create the desired reactionsand, hence, the dragout loss is reduced to an absolute minimum.

The fact that the salt approaches the strip in atomized condition, andunder high pressure, creates an almost instantaneous or flash-typeconversion of the oxide which means that the reaction takes place muchquicker and is believed to result in a greatly improved chemicalreaction. It is well known that a fast reaction in small increments isoften highly superior in its chemical action to a slow, prolongedreaction.

Not only is there a saving in salt because of the reduced wastage due todragout losses, but also a considerably greater amount of the saltenters into the reaction and gives a considerably greater degree ofcleaning action, or oxidizing action, than might otherwise be obtainedwhere a strip passes through a bath. Here, because of the 4 atomizedcondition of the salt impinging upon the strip under high pressure,there is a highly improved cleaning action as contrasted with the actionthat takes place in immersion baths.

The small amount of salt that is melted for use in the molten salt sprayprocess hereof is always free of buildups of complex metallic impuritiesand metallic salts such as chrome oxides, iron oxides, nickel oxides,manganese oxides, titanium oxides and others, as might be found inimmersion baths. While, to the best of our knowledge, no completeanalysis has been made as to how the buildup of these metallic saltsaffects descaling in salt baths, we are reasonably sure they do have adeleterious effect and that they impede the reaction.

The salt may be anhydrous. It might also contain some water; or it canbe a concentrated solution which is heated, pressurized and superheatedbefore it is sprayed onto the strip. In such case, flashing off thewater allows the salt to impinge as usual on the metal in anhydrousform.

Now having described the process hereof, reference is had to theappended drawings showing a typical apparatus.

In these drawings:

FIG. 1 shows diagrammatically one form of such apparatus in side view,with parts cut away for purposes of clarity.

FIG. 2 is a section on line 22 of FIG. 1.

The drawing shows an insulated steam box, mutfie, or oven 10, supportedon legs 11 and having entrance and exit slots 12-14. The metal strip 20being cleaned enters slot 12 after leaving an uncoiled means not shownand a pre-spray heating or annealing means 16, and leaves slot 14 on itsway to rinsing means, acid treatment means, heating means, recoilingmeans, etc., also not shown. In the straight through pass in chamber orhousing 10 from slot 12 to slot 14 the strip 20 is processed by theprocess of this application.

The strip is supported outside the box 10 by rollers 21, but issupported inside the box, not by contact with rollers therein, but byits being under tension between rollers 21.

The box It is inclined as shown so that products formed or deposited inthe box collect at a drain or discharge point 22 from where they areremoved by a suitable removal means, such as a condenser not shown,including an exhausting fan.

Solid salt, in the instance here shown is fed into hoppers 24 and passesinto melting chambers 26 where it is melted by steam admitted fromoutside the box 10 through a steam line 27, a steam manifold 28, andpipe terminals 30. In the steam lines connecting manifold 28 toterminals 30 are siphon spray nozzles 32 which siphon the molten saltfrom chambers 26 and spray it onto strip 20 immediately after the stripleaves annealing means 16, as soon thereafter as possible. The timing issuch that when the strip reaches the salt spray nozzles, the strip is ata maximum annealed temperature; but by the time it leaves exit slot 14and passes out of the steam atmosphere and into the room air atmosphere,it has cooled down to a point where it is not seriously affected by roomair.

Additional steam nozzles or lines 34 admit steam into the box to spraylive steam into the box to form a hot steam atmosphere therein at aboveatmospheric pressure.

Inside box 10 is a steam chest 36 supplied by lines 37 which maintains alive steam heated wall inside the box.

The following example illustrates the practice of this invention:

Example 1 A salt composition comprising 2 parts by weight of causticsoda, 1 part by weight sodium nitrate and 0.3 part by weight of sodiumchloride are heated with steam at 1000 F. to fusion, picked up as anatomized liquid, and sprayed in a high pressure blast upon both sides ofa stainless 8-18 chromium nickel steel strip as it leaves an annealingoven maintained at 1500 F. the metal strip moving at a rate of about 1ft. per minute and slowly cooling from the annealing temperature to thesteam cleaning temperature. The strip is then sprayed with steam aloneto clean off any adhering molten salts. The strip, as it leaves thesteam cleaning chamber box 10, as described above, is finally quenchedwith a spray of steam at a temperature of 250 F. At no point in thecontinuous passage from the annealing oven through the salt spray andquench is the steel strip contacted with rolls. It is finally dipped ina dilute 5% hydrochloric pickling bath as a bright dip to remove thereadily soluble lower oxide scale formed thereon in the salt spray oven,and is finally dried in air as clean, bright stainless-steel metal.

Now having described the embodiment here shown, reference should be hadto the claims which follow.

I claim: 1. In the continuous spray descaling of metal strip With moltensalt;

a descaling device comprising a housing enclosing a chamber; inlet andoutlet slots in opposite ends of said housing through Which hot metalstrip to be descaled may enter and leave While passing through saidchamber Without marring contact With the walls; means outside saidchamber for supporting and passing said metal strip entering and leavingsaid chamber through said inlet and outlet slots;

the length of the chamber being such, in relation to the Weight andstifiness of the metal strip and its speed of passage through thechamber and the location of the strip supporting means, that the stripis out of contact with the surfaces of the slot;

the chamber being free of members which might engage or make contactwith such strip in its normal passage through the chamber;

a supply of steam under pressure;

a supply of molten salt;

and nozzles in said chamber connected to both supplies, steam and moltensalt, for atomizing and spraying molten salt on to the strip passingtherethrough and for filling said chamber with steam under pressure toseal the inlet and outlet slots against entrance of outside air intosaid chamber.

2. Apparatus according to claim 1 including steam spray nozzles Withinthe chamber near the outlet slot, connected to the steam supply, forspraying salt-free steam on the strip as it approaches the outlet slot.

References Cited by the Examiner UNITED STATES PATENTS 1,818,041 8/31Chapin 68-54 1,907,429 5/ 33 Masland 685 .4 X 2,372,599 3/45 Nachtman.

CHARLES A. WILLMUTH, Primary Examiner. GEORGE J. NORTH, Examiner.

1. IN THE CONTINUOUS SPRAY DESCALING OF METAL STRIP WITH MOLTEN SALT; ADESCALING DEVICE COMPRISING A HOUSING ENCLOSING A CHAMBER; INLET ANDOUTLET SLOTS IN OPPOSITE ENDS OF SAID HOUSING THROUGH WHICH HOT METALSTRIP TO BE DESCALED MAY ENTER AND LEAVE WHILE PASSING THROUGH SAIDCHAMBER WITHOUT MARRING CONTACT WITH THE WALLS; MEANS OUTSIDE SAIDCHAMBER FOR SUPPORTING AND PASSING SAID METAL STRIP ENTERING AND LEAVINGSAID CHAMBER THROUGH SAID INLET AND OUTLET SLOTS; THE LENGTH OF THECHAMBER BEING SUCH, IN RELATION TO THE WEIGHT AND STIFFNESS OF THE METALSTRIP AND ITS SPEED OF PASSAGE THROUGH THE CHAMBER AND THE LOCATION OFTHE STRIP SUPPORTING MEANS, THAT THE STRIP IS OUT OF CONTACT WITH THESURFACES OF THE SLOT; THE CHAMBER BEING FREE OF MEMBERS WHICH MIGHTENGAGE OR MAKE CONTACT WITH SUCH STRIP IN ITS NORMAL PASSAGE THROUGH THECHAMBER; A SUPPLY OF STEAM UNDER PRESSURE; A SUPPLY OF MOLTEN SALT; ANDNOZZLES IN SAID CHAMBER CONNECTED TO BOTH SUPPLIES, STEAM AND MOLTENSALT, FOR ATOMIZING AND SPRAYING MOLTEN SALT ON TO THE STRIP PASSINGTHERETHROUGH AND FOR FILLING SAID CHAMBER WITH STEAM UNDER PRESSURE TOSEAL THE INLET AND OUTLET SLOTS AGAINST ENTRANCE OF OUTSIDE AIR INTOSAID CHAMBER.